Move an attribute between product and variant scope

Scope says where an attribute's values live on a product that has variants; on a product without variants the attribute is the product's own either way.

Moving to variant copies each product's value onto its variants that hold none (a variant's own value is never overwritten), then removes the product's value; a product without variants keeps its value as its own. Content Fabric writes per product cannot move to variant. Moving to product takes the value a product's variants agree on onto the product and removes it from the variants. Agreement is strict: every variant holds the same value, or none does. If any product disagrees, nothing moves and the answer is 409 with one errors entry per product (at most 20). Every value written is recorded in its history against the API key, and the change appears in the brand's activity.

A move rewrites up to 25,000 variants in one transaction, so it sits on the job budget.